This invention relates generally to automatic collimating systems and is concerned more particularly with a radiographic system having adjustable beam collimating means for protecting a person from overexposure to radiation.
It is well-known that internal organs of a human body, for example, may be examined by exposing a preselected region of the body to radiation, such as X-rays, for example, for a limited period of time. However, the radiation should be confined to the specific area of the body under examination in order to minimize exposure of the patient to the radiation. This objective is best achieved by precisely regulating the cross-sectional size of the beam irradiating the area of the body being examined.
One type of X-ray apparatus particularly suitable for achieving this objective is shown and described in U.S. Pat. No. 3,581,094 granted to L. F. Peyser et al and assigned to the assignee of this invention. The apparatus disclosed therein includes a beam limiting device or collimator having two orthogonally arranged pairs of opposing pivotal plates which form a rectangular aperture. The plates are made of X-ray absorbent material, such as lead, for example, and serve to regulate the cross-sectional size and shape of a beam passing through the aperture. In accordance with electrical signals produced by sensing devices suitably located in the apparatus, the rectangular aperture is adjusted to provide the beam with a cross-sectional size conforming substantially to the rectangular area of an X-ray film. Thus, when a patient is positioned between the beam limiting device and the X-ray film, only the portion of the patient's body selected for study should be irradiated and imaged on the film.
Another type of beam limiting device, which is more suitable for regulating the diametric size of an X-ray cone is shown and described in U.S. Pat. No. 3,448,270 granted to L. F. Peyser and assigned to the assignee of this invention. Briefly, this patent discloses a beam limiting device having an exit aperture defined by a thimble-like shutter comprising a plurality of X-ray absorbent leaves arranged longitudinally in partial overlapping relationship to form a frusto-conical structure. The leaves are pivotally mounted and simultaneously adjustable to move into greater or lesser overlapping relationship thereby defining the diametric size of a variable aperture at the small diameter end of the frusto-conical structure. Thus, this beam-limiting device may be adjusted to provide a cone of radiation passing through the structure with the proper diameter for impinging on a circular image receptor, such as the input screen assembly of an image intensifer tube, for example.
The described beam-limiting devices generally include means for adjusting the shutter aperture whereby an emerging X-ray beam is provided with a cross-sectional size which conforms substantially to the area of an image receptor. However, recent medical investigations completed by the United States Public Health Service indicate that the cross-sectional size of the X-ray beam, at the plane of the image receptor, should conform even more closely to the surface area of the image receptor than current practice permits. Thus, it has been found desirable to provide automatic means for setting the shutter aperture of the beam-limiting device in a manner which will comply with the recommendations of the United States Public Health Service. Automatic systems for precisely setting the shutter aperture by means of accurate mechanisms which overcome backlash and other forms of hysteresis have proved unsatisfactory, because of the undesirable cost involved and a tendency of the shutters to "hunt" when reaching a desired setting for the aperture. Attempts have been made to overcome this tendency to "hunt" by providing a "dead band" which renders the system insensitive to small amounts of "overshoot" when the shutters reach the desired setting. However, when the aperture is being adjusted, and the shutters reach the desired setting, this small amount of overshoot may result in the X-ray beam having a cross-sectional area which exceeds the allowed tolerances for conforming to the surface area of the image receptor.
Thus, it is advantageous and desirable to provide a radiographic system with a beam-limiting device and means for adjusting the shutter aperture of the device in a manner which overcomes backlash and other forms of mechanical and electronic hysteresis without resorting to prohibitively expensive mechanisms and electronic controls for achieving the desired accuracy.